Effect of Al-Cu-Fe Quasicrystal Particles on the Reinforcement of a Polymer–Matrix Composite: From Surface to Mechanical Properties

We examined the effect of Al59Cu25Fe13B3 (at.%) quasicrystalline (QC) reinforcement particles on the mechanical and surface properties of a polymer-matrix composite by applying a technical polymer polyphthalamide (PPA). The observed increase in the tensile Young’s modulus ranged from 1810 MPa for the pure polymer to 4114 MPa for the composite with a QC filling of 35 vol.%. The elongation at fracture decreased with the filling fraction, being equal to 16.9% for a pure polymer and dropping to 4.8% for the composite with a QC filling of 35 vol.%. The same trend was noticeable with flexural Young’s modulus, which ranged from 100 MPa for a pure polymer to 125.5 MPa for the composite with 35 vol.% of QC. It was found that the increase in the mechanical strength led to a simultaneous increase of brittleness, which was reflected in a decrease of the impact strength for a pure polymer from 98.5 kJ/m2 to 42.4 kJ/m2 for composites with a QC filling of 35 vol.%. In contrast, when filled with 5 vol.% of QC, the impact strength increased by 8%. The friction coefficient against 100C6 steel dropped from 0.15 for pure PPA down to 0.10 for 5 vol.% of the QC filling, followed by an increase to 0.26 for further QC fillings up to 35 vol.%. Interestingly, a local minimum of friction was achieved at filling factors between 5 to 20 vol.% of QC. Independently, a clear surfenergy minimum was also found for the composite material with 20 vol.% of QC filling associated with a net drop in the polar component of the surfenergy. Surfenergy refers to the surface energy related to the top of the oxide layer under ambient conditions. We hypothesise that this is related to the percolation threshold at about 13 vol.% QC, reflected in the observed behaviour of both the friction coefficient and surfenergy. For the pure QC annealed in air for 1 h at 500 °C significant wear tracks were observed accompanied by a wear debris formation. On the other hand, a pure polymer exhibited slightly visible wear tracks with no apparent debris formation, and for the composites with different QC filling factors, the wear traces were barely visible with negligible debris formation.